1
A Study of Corrosive Reactions with the TG/
MS: Using Pure H2S to Sulfuret TiO2
Application Note
C-221
Abstract
The sulfurization of Titanium Dioxide
TiO2 by pure Hydrogen Sulfide
H2S was successfully monitored
using a simultaneous Thermogravimetry
TGMass Spectrometer
MS system. While confirming the
progress of the reaction from TG
and MS data, experimental results
also showed that such a reaction was
diffusion controlled.
Introduction
For materials scientists, it is necessary
to study the endurance of materials
under different environments.1-4
These include temperature, temperature
cycling, pH, pressure, corrosive
gases, etc. A common approach involves
studying the raw materials
under “accelerated“ experimental
conditions. The “accelerated“ condition
means that the material is put
in an environment that will cause it
to corrode, break, or degrade at a
faster rate than if run under actual
use conditions. By obtaining the information
about the material’s reaction
mechanism, the endurance of
the material can be projected more
cost effectively.
For metallurgists, many interesting
reactions involve corrosive gases.
However, corrosiondecomposition
mechanisms are very difficult
to study when reaction gases like
H2S, SO2, HCl, HF, H2SO4, or halogen
gases, are utilized. Therefore, it
is important to have instruments
that are resistant to the effects of
these types of reactive gases.
Thermogravimetry TG has been
extensively used for the study of the
gas-solid reactions, by monitoring
the weight change of the sample as
a function of temperature and
time.5-9 It will provide information
about the reaction mechanism, reaction
rate,kinetic parameters.
By attaching an Evolved Gas Analyzer
EGA to the vent port of the TG
instrument, the evolved gases can be
identified. One of the most commonly
used TG/EGA techniques is a TG
coupled with a Mass Spectrometer
MS. A unique coupling system is
used to reduce the effect of the reaction
gas on the sensitive MS system.
With the information provided by
the MS data, the reaction mechanism
and kinetic parameters can be further
supportedexplained. It’s especially
useful when a mixture of gases
is evolved from the reactions.10-12
In order to demonstrate the capability
of a TG/MS system under a
corrosive environment, this paper
will illustrate the use of pure hydrogen
sulfide as the reaction gas to
sulfuret a sample. The process is
similar to the reaction of oxygen
with a metal to form a metal oxide.
The sulfurization process requires
heat to initiate the reactionis
characterized by a small increase in
the sample’s weightthe evolution
of water or other gases. For this paper,
the reaction is:
TiO2s + 2 H2Sg TiS2s + 2 H2Og
Such a reaction was monitored by
the TG/MS system, along with other
typical reactions that are normally
used for demonstrating the capabilities
of a TG/MS system.
Experimental Set-up
Apparatus
Experimental apparatus was a
Thermo Scientific Chan TG/MS synergy
system. The system consists of
a Thermo Scientific Cahn TG-131
system, a Finnigan Mass Spectroscopy
MS Automass II system, and
TG/MS interface. The Thermo
Scientific Cahn TGA has a sample
capacity of 100 grams, with a sensitivity
of 1 mg. The TG is designed
in such a way that the balance is protected
by an inert purge gas, while the
sample is exposed to the desirable
reaction gases. Since it’s also vacuum
tight, the system is capable of handling
a corrosive gas without losing
the sensitivityaccuracy of the
weighing mechanism.
The MS has a mass range of 4 to
1000 amu. The dual turbo-pumps
in the system provided a very high
intake flow ratethe shortest
residence time. With the vacuum
tight design, which is minimum requirement
for any MS system, it is
also capable of handling a wide variety
of corrosive gases.
The synergy style TG/MS interface is
designed in such a way that the
effluents from the sample located in
the reactor tube of the TG are immediay
analyzed by the MS. The TG/
MS interface is kept at 150 °C during
the experiments to prevent any condensation
and MS spectra were collected
simultaneously during the TG
experiment.
MaterialsExperimental
Parameters
Four different reactions were monitored
by the TG/MS system. The
decomposition of calcium oxalate
monohydrate from Aldrich-Sigma,
Part #28984-1 under nitrogen, the
burning of a bituminous coal from
VWR Scientific Products, Part
#WL7073Z-07 sample under air,
the decomposition of a polyimide
resin from Aldrich-Sigma, Part
#18464-0 under nitrogen,the
sulfurization of titanium dioxide
from Aldrich-Sigma, 99.999%
purity under hydrogen sulfide.
Sample 1, Calcium oxalate monohydrate,
was heated from room temperature
to 1000 °C at a heating rate
of 10 C/min with a nitrogen reaction
gas flow rate of 50 cc/min. Sample 2,
[
Δ
Dun Chen, Thermo Fisher Scientific, Process Instruments, Newington, USA
2
bituminous coal was heated from
ambient temperature to 1000 °C at
a heating rate of 10 °C/min, under
a 50 cc/min flow of air. The sample
was held at 1000 °C for 15 minutes
to ensure the completion of the reaction.
A polyimide resin sample was
heated from room temperature to
950 °C at a heating rate of 10 °C/
min in nitrogen at a flow rate of 50
cc/min. The titanium dioxide sample
was heated from ambient temperature
to 800 °C at a heating rate of
10 °C/min,held at 800 °C for
20 minutes. In this sample, hydrogen
sulfide was used as the reaction gas
at a flow rate of 50 cc/min. The experimental
parameters for each sample
are summarized in Table 1.
ResultsDiscussion
Calcium oxalate monohydrate has
been used commonly for the testing
of TG/MS systems, because it will
show three distinguishable weight
loss stepsgive off a different
component at each step. The decomposition
steps of calcium oxalate
hydrate are expressed as follows:
CaC2O4
.H2Os CaC2O4s + H2Og
CaC2O4s CaCO3s + COg
CaCO3s CaOs + CO2g
Figure 1 shows the TG/MS results
for heating of calcium oxalate monohydrate
under nitrogen. The TG
curve shows that there are three
weight loss steps, the 1st one at
around 200 °C, the 2nd one at around
500 °C,the 3rd one at around
850 °C. They correspond to the
above three decomposition steps.
The MS data confirmed the above
steps by monitoring the evolved
gases. Since nitrogen was used as the
reaction gas, which has the same
mass as that of carbon monoxide,
28, the evolution of carbon monoxide
was blanked by this reaction gas.
The small amount of carbon dioxide
detected on the 2nd weight loss step
was caused by the small amount of
oxygen presented in the reaction gas,
which caused the oxidation of carbon
monoxide into carbon dioxide.
The TG/MS results for the burning
of the bituminous coal sample are
shown in Figure 2. It can be seen
from the TG curve that there were
two weight loss steps. One occurred
at a low temperatureanother at
a higher temperature. From the MS
data, it’s confirmed that the first
Δ
Δ
Δ
Table 1. Experimental Parameters
Figure 1. Sample 1, Calcium Oxalate Monohydrate under Nitrogen
RT to 1000 °C at a rate of
10 °C/min.
RT to 1000 °C at a rate of
10 °C/min,15 minutes
isotherm at 1000 °C.
No Sample ID
bituminous coal
Reaction Gas Flow Rate Heating Profile
RT to 950 °C at a rate of
10 °C/min.
RT to 800 °C at a rate of
10 °C/min,20 minutes
isotherm at 800 °C.
titanium diopxide
1
2
3
4
CaC2O4
.H2O N2 50 cc/min
50 cc/min
50 cc/min
50 cc/min
air
N2 polyimide resin
H2S
weight loss step was caused by the
loss of moisture in the sample. The
2nd weight loss step was the burning
of the coal, due to the detection of
carbon dioxide by the MS. However,
since the sample is a mixture containing
other compounds, it also gave
off watersulfur dioxide during
oxidation. Therefore, the reactions
that occurred during the burning of
bituminous coal sample were most
likely:
Cs + O2g CO2g
Ss + O2g SO2g
Other componentss + O2g
CO2g + H2Og
The above two reactions were easy
to followare often used to verify
the operation of a TG/MS system.
However, when a complex sample
is analyzed, such as polyimide resin,
the TG/MS data will require more
sophisticated interpretation. Figure 3
shows the TG/MS data for a
polyimide resin sample under a nitrogen
environment. It can be seen
from the TG scan that there were two
weight losses. The first one occurred
at around 100 °Canother
around 550 °C. Based upon the MS
results, water was released during
the first weight loss step. The second
weight loss was due to the decomposition
of the polyimide resin
sample. During this decomposition
Δ
Δ
Δ
Figure 2. Sample 2, Bituminous Coal Under Air
Temperature deg C
Time hrs:min
3
step, watercarbon dioxide were
released, along with other compounds.
In order to properly interpret
the data, it is necessary to obtain
more structural information about
the sample. Additional experiments
using TG/FTIRTG/GC/FTIR/
MS will be madereported in a
subsequent paper.
The above reactions show the capability
of the TG/MS when using noncorrosive
reaction gases. The next
phase of this study is to monitor a
corrosive gas reaction. Figure 4
shows the TG/MS results for the
sulfurization of titanium dioxide
under a pure hydrogen sulfide environment.
TiO2s + 2 H2Sg TiS2s + 2 H2Og
It can be seen, from Figure 4, that
there was an initial weight loss during
the first 25 minutes. It was believed
to be related to the loss of the absorbed
moisture. After about 1 hour
and 10 minutes into the run, at 660
°C, the sulfidization reaction occurred.
From the derivative TG DTG curve,
it can be seen that the reaction
reached its maximum rate at around
1 hour23 minutes. After that,
the reaction slowed down. It appeared
from the shape of the DTG
curve that the reaction occurred
aggressively during the initial stage.
It was suspected due to the conversion
of TiO2 to TiS2 on the surface
of the sample, the continued reaction
was inhibitedmay have followed
a diffusion mechanism. This was
confirmed by examining the residue
after the run, which had a layer of
yellow TiS2 on the surface of the
sample. Upon crushing the sample,
an internal white TiO2 layer was
exposed.
The MS analysis result for the evolution
of water in Figure 4 also showed
a very similar shape as that of the
DTG curve. This is in excellent agreement
with the TG analysis results,
and further confirmed that TiO2 was
sulfidized to TiS2 under a diffusion
reaction mechanism.
Conclusions
Experimental results proved that
Thermo Scientific Cahn TG/MS
system can not only monitor the
reaction under regular inert and
oxidizing environments, but also
study reactions under corrosive
Figure 3. Sample 3. Polyimide Resin under Nitrogen
Figure 4. Sample 4. Titanium Dioxide under Pure Hydrogen Sulfide
reaction gas environments. While the TG data provide the information
about the reaction rate, reaction mechanism,kinetics parameters, MS
data can further support the TG analysis results by monitoring the evolved
gases as an aid in determining the reaction mechanisms.
References
1. Pasquevich, D.M.,Carneiro, A., Thermochimica Acta, 1989, 156,
pp 275-283.
2. Soleiman, M. K.,Rao, Y. K., Metall. Trans. B, 1987, 18, 459.
3. Grob, B.,Richarz, W., Metall. Trans. B.,1984, 15, 529.
4. cNallan, M. J.,Liang, W. W., J. Am. Ceram. Soc., 1981, 64, 302.
5. Turi, E. A., Thermal Characterization of Polymeric Materials, 2nd ed.,
Academic Press, 1997, vols 12.
6. Dollimore, D., Analytical Chemistry, 1998, 70, 27R-36R.
7. Dollimore, D., in Analytical Instrumentation Handbook, 2nd ed.,
Ewing, G. W., Ed., Marcel Dekker, New York, 1997, Chapter 17, p 947.
8. Brown, M. E., J. Therm. Analysis, 1997, 49, 17.
9. Flynn, J. H., Thermochimica Acta, 1997, 300, 83.
10. Bi, M., Li, H., Pan, W. P., Lloyd, W. G.,Davis, B. H.,
Thermochimica Acta, 1996, 284, 153.
11. Wilkie, C., Mittleman, M. L., Adv. Chem. Ser., 1993, 236, 677.
12. Marini, A., Berbenni, V., Capsoni, D., Riccardi, R., Zerlia, T., Appl.
Spectrosc., 1994, 48, 1468.
Δ
Time hrs:min
Temperature C
4
Thermo Fisher
Scientific
Process Instruments
USA
25 Nimble Hill Rd.
Newington, NH 03801
. 603 436 9444
info.mc.us@thermofisher.com
www.thermo.com/cahn
C221_05.06.07
© 2007/06 Thermo Fisher
Scientific· All rights reserved ·
This document is for informational
purposes onlyis subject to
change without notice.